Electron lens



. D17, 1946. of; H, KLEMPERE 2,412,681

, ELEcTRoN LENS l Filed Mamh s1, 1944l .f7 f 3 j" .4g/Q ,n F5914 t4-Fgr/. y m M., 550% l j f j 1v1 4 LVZ F Z 'l (Ittorneg Patented Dec.17, 1946 ELECTRON LENS Otto Ernst Heinrich Klemperer, Iver, England, as-

signor to Electric and Musical Industries, Ltd.,

a British corporation Application March 31, 1944, Serial No. 528,994 InGreat Britain April 8, 1942 (Cl. Z50-1.60)

13 Claims. l

The present invention relates to electrode systems for producingelectron lenses such as are employed in cathode ray tubes, electrondischarge valves and other electron discharge devices for focusingelectron beams and to electron discharge v. devices including suchsystems.

An electrode system for producing an electron lens is usually disposedsymmetrically about an axis and affords a passage through which the beamto be acted on is projected. For example, the electrode system maycomprise two axially aligned plane diaphragm electrodes having centralapertures defining said passage or tubular electrodes embracing saidpassage with their ends or transverse edges included in planesperpendicular to the axis, Thus, Figures 1A and 1B of the accompanyingdrawing are each diagrammatic side elevational Views of known systems oftubular electrodes, each comprising two aligned tubular electrodes A andB respectively, through which the beam to be acted upon is projected.With such systems, if electrodes A and B are maintained at differentpotentials V1, V2, respectively, the electron lens so produced will beconverging whichever electrode is the more positive and whether or notan electron beam is projected from left to right or from right to leftthrough the system. The focal length of the lens will depend on theratio of the voltages applied to the electrodes A and B with respect tothe cathode of the beam operated upon. As illustrated in Figure 1A,electrodes A and B may lbe of the same diameter so that the edges of theelectrodes are in registry, the electrodes being separated by a gap Ghaving an axial length sumcient to provide an adequate insulationbetween the two electrodes. In this case the electron lens isconstituted by the electrostatic eld produced about the gap G and themid-suriace of the lens,which is an arbitrarily selected surface ofreference located in the manner described, for example, in the CambridgePhysical Tract Electron opties by O. Klemperer, Cambridge UniversityPress, i939, is constituted by the surface midway between the surfacesincluding the inner edges of electrodes A and B, namely the edges of theelectrodes A and B on either side of the gap G. As these surfaces areplane the midesurface is a plane and is indicated by the chain dottedstraight line M of Figure 1A. If the electrodes A and E are of differentdiameters they are preferably disposed with the end of one electrodeinserted within theV other as electrode illustrated inFigure 1B. In thiscase the lens is constituted l by the'electrostaticiield producedaboutthev end,

of the smaller diameter electrode (A in Figure 1B) embraced by thelarger diameter electrode l (B in Figure 1B), and the mid-surface of thelens is defined as the surface including the inner edge or edges cf theaforesaid end of the smaller Thus, in the case shown in diameterelectrodes. Figure 1B the mid-surface of the lens is constituted by theplane of the aforesaid end of the smaller` diameter electrode, thisplane being represented by the line M in Figure 1B. The electrodes A andB may be of circular, rectangu lar, square, elliptical or other desiredcross-section having axial symmetry, or they may each comprise a pair ofoppositely disposed elements, for example of plate form.

In the case of an electron lens system com prising a pair oi diaphragmelectrodes the midsurface of the lens is defined, as the surface midwayIbetween the facing surfaces of the diaphragm electrodes. Thus, in thecase of the hitherto known arrangements including plane diaphragms themid-surface is again also a plane;

In cases where the electrode system comprises more than two electrodesthe field produced is equivalent to that of several pairs incombination, and the system may be treated as producing a combination ofseparate lenses each constituted between one electrode and the next insuccession along the axis. In the case of a system comprising threeelectrodes for forming a saddle eld lens, the mid-surface may be denedas the surface midway between the surfaces including the inner edges orfacing surfaces as the case may be of the outermost electrodes, but thearrangement can also be regarded as composed of a pair of lensesconstituted by the electrostatic fields in the gaps between theelectrodes.

As far as I am aware, in the prior construction of electron lens systemsemploying only electrodes which afford an unobstructed passage for thebeam to be acted upon, such as tubular or diaphragm electrodes orcombinations of tubular or diaphragm electrodes, the edges oflens-forming tubular electrodes in the lens ileld, herein referred to asthe inner edges, or the facing surfaces of the lens-forming electrodeshave been bounded by planes and consequently the midsurfaces of thelenses have been planes. I am aware, however, that electrode systems forforming electron lenses have been proposed including curved electricallyconducting mesh elements disposed across the path of the beam to beacted on, and in some suchconstructions the mesh lenses obtained wouldhave curved mid-surfaces.

However, electrode systems employing mesh elements have thedisadvantages that the mesh elements intercept electrons in the beam andit is diflicult to obtain satisfactory focusing. Moreover, asl far as I-am aware, the advantages to be obtained by employing lenses with curvedinidsurfaces have never been appreciated.

The present invention arises out of an investigation for the purpose ofproviding improved electrode systems for producing electron lenses` forfocusing an electron beam in a, line, for example as required in certainkinds of electron discharge valves for high frequency operation, forexample, in devices of the kind known as Klystrons.

According to one feature of the present invention an electrode systemfor formingan electron lens is provided compri-sing two alignedcooperating electrodes affording an unobstructed passage for anelectronbeam to be acted upon, and having the inner edge or edges or eachelectrode presented to the inner edge or edges of the other, said edgeor edges being so curved that the mid-plane of said lens is curved.

According to another feature of the present invention an electrodesystem for forming an electron len-s is provided comprising two alignedcol-operating electrodes affording an unobstructed passage for anelectron beam and of which one electrode is of smaller diameter than theother and'has one end inserted within the other, the edge of said endbeing so curved that the midplane of said lens is curved.

According to a further feature of the present invention an electrodesystem for forming an electron lens is provided comprising. a pair ofapertured diaphragm electrodes disposed with their apertures aligned toafford an unobstructed passage for Van electron' beam, said electrodeshaving their facing surfaces so curved that the mid-plane of said lensis curved.

The invention also includes electron discharge apparatus having anelectrode system as above set forth and means for projecting an electronbeam through said passage and means for maintaining the electrodes ofsaid system at such potentials that said beam i-s acted upon in adesired manner.

Among the objects of the invention are to provide an improved method ofand means for focusing an electron beam.

Another object is to provide an improved method of and means forreducing aberration in an electron lens.

A further object is to provide an improved i method of and means forderiving a line image of anelectron source.

An additional object of the invention is to prov vi-de an improvedelectron lens having a curved `Figures 2 to i0 of the accompanyingdrawing, of

which Figure 2 is a perspective View of an electrode system according tothe invention,

Figure 3 -isa perspective view of an electrode for use in a furtherelectrode system according to the invention,

Figures 4A and 4B are plan and elevational view-s respectively of anelectrode system according to the invention including cylindricalelectrodes,

Figure 4C is a perspective view of one of the electrodes shown inFigures 4A and 4B,

Figure 4D is a plane development of the electrode ofv Figure 4C,

Figures 5A and 5B are a perspective view and a plane developmentrespectively of a further electrode of cylindrical cross-section for usein a system according to the invention,

Figures 6 and 7 show two electrode systems formed in accordance with theinvention, and each of which comprises two tubular electrodes, one ofwhich is of smaller diameter than the other and has one end insertedwithin the other electrode in the manner described with reference toFigure 1B above,

Figure 8 illustrates a construction of an electrode for use in thesystem in accordance with the invention of which the 1ens-formingelements are of diaphragm form, and

Figures 9 and 10 show two electrode systems formed in accordance withthe invention and each including more than two lens-forming electrodes.

In Figure 2 of the drawing an electrode system is shown which issuitable for focusing an elec- Jtron beam at a line perpendicular to theaxis of the System. Such a system may be employed, for example, forfocusing a, fiat ribbon-shaped beam.

The system shown in Figure 2 comprises a pair cf electrodes A and Baligned and having their sides in registry and aifording a passage ofuniform rectangular cross-section throughout their length. Theelectrodes are separated by a gap G to isolate them electrically fromone another. The electrode A has horizontal sides l and 2 and verticalside-s 3 and li, while the electrode B has horizontal sides 5 and E andvertical sides and 8. The horizontal sides of the electrodes A and Barerepresented as being wider than the vertical sides, but for someapplications, the sides may all be of the same width so that thecross-section of the electrodes is square or the vertical` sides may beWider than the horizontal sides.

if the electro-des A and B are maintained at different potentials, theelectron lens produced is analogous to twov crossed cylindrical opticallenses, one lens corresponding in effect to the eld due to the sides E,2 and 5, S of the electrodes A and E and serving to focus a projectedelectron beam at a line in the horizontal plane through the axis of thesystem, and the other lens corresponding in effect to the field due tothe narrow sides 3, 4 and "l, 8 of the electrodes A and B and tending tofocus the beam at a line in the vertical plane through the aforesaidaxis.

`In accordancewith the present invention, :the inner ends of thevertical sides 3, i and l, 8 of the electrodes A and B are formed withcurved edges to constitute an electron lens having a curved mid-surface.Thus, in the example illustrated in Figure 2 the curved edges are cut toconform to cylindrical surfaces of radius R having their axesperpendicular to the axis of the electrode system, the curved edges onelectrode A being recessed and the curved edges on electrode B beingconvex and complementary to those of electrode A. The mid-surface of theelectron lens will then be a cylindrical surface of radius R located inthe middle of the gap G.

,Such a system is found to give better focusing semicircular. propertiesof the lens, the curvature of the midsurface also controls to someextent the focal the electrodes.

having au the innereedges straight and win handle satisfactorily a beamfilling a greater part of the cross `section of the electrode systrodesA and B so that the edges thereof are Besides improving the focusingVlength of the lens.

If theV electrode B is maintained at a higher positive potential thanthe electrode A, a parallel will cause a beam projected through it ineither direction to diverge.

As already pointed out, the vertical sides 3, 4 and l, 8 of theelectrodes A and B of Figure 2 have a slight focusing action on anelectron beam passing through the electrodes. 'I'hey also cause thefocal line obtained when a at, ribbonshaped beam is projected throughthe system to be curved especially in the vicinity of the sides. Thesedifficulties can be avoided by curving the inner edges of the horizontalsides I, 2 and 5, 6 of electrodes A and B. For example, Figure 3 shows aperspective View of an electrode B' which corresponds to the electrode Bof Figures 1 and 2, but which has the edges of its wide sides 5, 6 cutaway or recessed toconforrn to a cylinder of radius RI. If the electrodeB is employed with a co-operating electrode similar to the elec-- trodeA of Figure 2 but having the .inner edges of its horizontal sides suchas I, 2 convexly curved to conform to a cylinder of radius RI, then ifthe value of RI is correctly chosen an electron lens may be producedwhich is effectively the equivalent of a 'single pair of wide plate formelements corresponding to the sides I, 2 and 5, 6 of electrodes A and Bwithout the vertical sides 3, 4 and '1, B, though the vertical sides arestill present to shield the path of the beam through The radius RI maybe made quite small, for example, a simple, but quite effective,construction is afforded by making the edges of the horizontal sidessemicircular, the curvature of the edges of the horizontal sides beingdetermined by the width thereof.

In the systems described with reference to Figures 2 and 3 of thedrawing the curvature of the midsurface of the lens is identical withthat of the surface in which the curved inner edges of the lens-formingelectrodes A and B or A and B are located, but it is not essential thatthis should -be so. Thus the curvature of the inner edgesof theelectrodes may be made to diier so that the curvature of the mid-surfaceis different from that of the edges of either electrode. For example, ifthe radii of the registering edges of the electrodes differ by an amountequal to the width of the gap G, the radius of the mid-surface will beequal to the mean of the radii of the adjacent edges. Furthermore, theinner edges of the electrodes A and B need not conform-Ito cylindricalsurfaces. For example, in order to correct for aperture errorcorresponding to' spherical aberration in an optical lens, it may bedesirable to cause the radius of `curvature of the mid-surfaceto begreater at the center of the lens neld about' the axislof the.`

l*lens forming electrode system than it is on the edgesthereof.

Figures 4A to 4D of the drawing illustrate the applicationV of theinvention to a system of electrodes in which the electrodes arecylindrical and afford a passage of uniform circular cross-sectionthroughout. This system, like the systems described with reference toFigures 2 and 3 is intended `for focusing a ribbon shaped beam to aline.

Referring to Figures 4A and 4B it will be Iseen that the electrodesystem shown comprises a pair of electrodes A and B having their edgesformed with diametrically opposed projecting portions II, I 2 or I3, I4as the case may be, the edges being complementary. The electrodes A andB" are identical in form, the single electrode B" being shown in Figure4C and a plane development of this electrode is shown in Figure 4D. Fromthe latter figure it will be Iseen that the projecting portions I3 andI4 are formed by cutting the edge of the electrode to conform to aseries of semicircular arcs I 5, I6, II and I8 each of radius equal toan eighth of the circumference'of the tube, the arcs being alternativelyconvex and recessed. The electrodes may be formed by severing a singleflat sheet of material along the line of the arcs I5 to I8 and rollingthe two severed portions of the sheet to provide the electrodes.

The system of Figures 4A and 4B will focus a beam having a diameterequal to the radius Rr of the tubular electrodes at a line withoutgiving rise to any appreciable aberration and with quite small ratiosbetween the voltages on the electrodes. For example, to produce a linefocus at a distance of ve `tirnes the tube radius from the principalplane of the lens system the voltage ratio is only about 1.4 whereas astraight edge two-tube lens requires a voltage ratio of about six togive the s arne focal length.

However, the length of the focal line may be greater than the originaldiameter of the beam and the beam may be divergent in directionsparallel to the plane, and, in fact, with the construction of electrodesdescribed with reference to Figures 4C and 4D, the divergency of thefocused beam is such that the focal line will have a length of the orderof twice the diameter of the original beam. If it is desired that thebeam shall not be divergent, or should be of different divergency theinner edges of electrodes A" and B" can be shaped accordingly. Forexample, if, instead of cutting the inner edges of the electrodes toconform to semcircular arcs of radius (or one eighth the circumferenceof the tube) as described with reference to Figure 4D, the arcs are notmade semi-circular, but are given a radius 21rRr approximately, a systemwhich produces practically no divergency of the beam will be produced.The focal length of the system will be increased however, and also theaberration propoint focus wouldbe obtained. From this it will beappreciated that if the radii of the aforesaid arcs are greater than21rRT the beam will con- VergeV after passing through the lens systemsAsuitable for use in cathode Yray guns. as to whether a cylindricalelectrode such as A lof Figure 6 or a square section electrode such as,'A of Figure 7 is employed will depend on which electrode is the moresuitable for incorporation in posed at different distancesfromthemid-surface and in planes at right angles to each other.

In some cases a satisfactory focusing of a rib-- bon shaped beam may beobtained with complementary cylindrical tubular electrodes formed in themanner illustrated at A" in Figures A and 5B with straight edge portionsl2l and 22 alternately with arcuate portions either convex as shown at23 and 24 in Figures 5A and 5B or concave. trodes constructed in thismanner is no-tthought to have a very wide application. It is believedthat having regard to the description already given with regard to thesystem of Figures 4A and 4B, the Inode of operation of a systememploying electrodes formed in the manner indicated with reference toFigures 5A and 5B will be understood without difficulty.

Figures 6 and 7 of the drawing show theapplication of the invention tolens systems of the kind described above with reference to Figure 1B, inwhich the lens eld is formed at the end of a tubular electrode insertedwithinanother electrodeof larger diameter. Thus, in the example shown inFigure 6, the system is constituted by co-operating cylindricalelectrodes A and C, the electrode A" having one end formed withprojecting portions Il and l2 in the manner described with reference toFigures 4C and 4D above, and having this end inserted within the largerdiameter electrode C, shown partly broken away to show the `dispositionof parts within its interior. rheelectrode-C may be constituted by aconducting coating on the neckof a cathode ray tube for example. ofFigure 6 carries a conducting flange 25 for effectively closing the endIof `electrode C so as to shield the lens field from field penetrationthrough the gap between the electrodes. A suitable gap must bemaintained between the flange i and the electrode C for insulatingpurposes. With the arrangement o-f 'Figure A6 the lens field obtained issimilar in effect to that produced by the system of Figures 4A `and 4B,which has been described in detail above.

Figure 7 shows an arrangement similar to that shown in Figure 6 but inwhich the cylindrical electrode A" is replaced by an electrode A ofsquare cross-section, and the inner end of which is formed withsemi-circular edges in the manner described with reference to Figure 3above. The effect of the square section electrode A is substantially thesame as that of a tubular electrode such as A" of Figure 6 of a diameterequal to the length of the side of the square cross-section, and i itwill be appreciated that the curvatures of the inner ends of theelectrodes A and A of Figures 6 and 7 can be varied 'in the mannerindicated with-.reference to Figures 4A and'4B above, the lens beingdiverging or converging or substantially ineifective in planes parallelvto the plane of symmetry between the projecting portions of theelectrode according to the curvature of the edges of electrode A or A"as the case may be.

The construction shown in Figures 6' and 7 are particularly free fromerrors due to misalignment of the lens forming electrodes yand areeminently The choice the particular device in which it is employed. Bothconstructions give lens fields with excellent qualitiesin respect'offreedom from aberration,

However, a lens system formed of elec- Preferably the electrode A" .i

the construction of Figure 6 being particularly .by a diaphragm 28,constituting the lens-forming element, the diaphragm being curved toconform to a cylindrical surface and havingv a slot 29 eX- tendingparallel to its curved edges. This electrode may be arranged toco-operate with a similarly formed electrode but with the end carryingthe diaphragm recessed and curved in a manner complementary to thecurvature of the end of the electrode D. With such an arrangement, if aiiat ribbon-shaped beam is projected axially through the system with itslonger dimension in the direction parallel to the edges of the slotssuch as 29 in the diaphragm, and the electrodes are maintained atdifferent potentials, the beam will be focused at a short line.

In'some applications, diaphragms'such as 28 might be constituted simplyby elements in the form of strips separated by a gap corresponding tothe slot 2:23, and the shield portion 2! might be omitted.

Figures 9 and 10 show the electrode systems for producing a plurality oflenses having mid-surfaces curved in accordance with the invention.

The arrangement of Figure 7 includes three electrodes 3E?, 3|, 32,separated by gaps 33, 34, the two mid-surfaces of which are oppositelycurved. The potentials V1, V2, V3, respectively, of the electrodes B, 3land 32 may be arranged to increase or decrease successively, or theelectrode 3i may be at a more positive or less positive potential thaneither of the electrodes 30 and 32. If the potential of the electrode 3|is lower than both of the electrodes 3i) and 32, a saddle field isproduced.l In Figure 8'an electrode system is shown comprising fiveelectrodes 35, 36, 3?, 33, 39, separated by gaps 4U, 4l, 42, 43, theelectrodes being formed with curved edges in accordance with theinvention. In operation the potentials V1, V2, Va, V4, V5, respectively,of these electrodes may be arranged to increase or decrease successivelyto provide a series of similar lenses.

Having now particularly described and ascertained the nature of my saidinvention and in what manner the same is to be performed, I declare thatwhat I claim is:

l. .An electrode system for forming an electron lens comprising twoaligned co-operating electrodes "affording an unobstructed passage foran electron beam to be acted upon, and having the inner edges of eachelectrode presented to the adjacent inner edges of the other of saidelectrodes, said edges being differently lcurved with Y respect to eachother to provide a curved lens mid-plane, the curvature of saidmid-plane being related to the shape of said electrodes to minimizespherical aberration in said lens.

2. An electrode system of the type described in claim l characterized inthatsaid adjacent inner edges of said electrodesare concentricallycurved.

3. An electrode system of the type described in claim l characterized inthat said adjacent inner edges of said electrodes are complementarilycurved and in registry.

4. An electrode system of the type described in claim 1 wherein each ofsaid electrodes comprises a pair of parallel disposed plane elementsdisposed symmetrically on opposite sides of said passage, said adjacentinner edges of each of said pairs of said elements being curved t0determine said mid-plane curvature.

5. An electrode system for forming an electron lens comprising twoaligned co-operating electrodes affording an unobstructed passage for anelectron beam and of which one electrode is of smaller diameter than theother and has one end inserted within the other, the edge of saidinserted end of said smaller diameter electrode being curved to providea curved lens mid-plane, the curvature of said mid-plane being relatedto the shape of said electrodes to minimize spherical aberration in saidlens.

6. An electrode system of the type described in claim 1 wherein saidelectrodes are of cylindrical diierently curved with respect to eachother to provide a curved lens mid-plane, the curvature of saidmid-plane being related to the shape of said electrodes to minimizespherical aberration in said lens.

9. An electrode system of the type described in claim 1 including meansfor applying focusing potentials to said electrodes, and wherein saidlens mid-plane curvature provides an electron image forming a linetransverse to the axis of said electron beam.

10. An electrode system of the type described in claim 5 including meansfor applying focusing potentials to said electrodes, and wherein saidlens mid-plane curvature provides an electron image forming a linetransverse to the axis of said electron beam.

l1. An electrode system of the type described in claim 8 including meansfor applying focusing potentials to said electrodes, and wherein saidlens midplane curvature provides an electron image forming a linetransverse to the axis of said electron beam.

12. An electrode system for forming a cornpound electron lens systemcomprising a plurality of aligned cooperating electrodes affording anun.. obstructed passage for an electron beam to be acted upon and havingthe inner edges of each electrode presented to the adjacent inner edgesof another of said electrodes, said edges being differently curved withrespect to each other to provide a curved lens mid-plane between each ofsaid adjacent electrodes, the curvature of said mid-plane being relatedto the shape of said electrodes to minimize spherical aberration in saidlens. i

13. An electrode system of the type described in claim 12 includingmeans for applying focusing potentials to said electrodes, and whereinsaid lens mid-plane curvature provides an electron image forming a linetransverse to the axis of said electron beam.

OTTO ERNST HEINRICH KLEMPERER.

